Method and apparatus for identifying and acquiring preferred wireless communications systems

ABSTRACT

In a mobile station having an acquired wireless communications system, the mobile station performs preliminary searches for more desirable wireless communications systems in between paging channel and quick paging channel assigned slots, while successfully monitoring the mobile station&#39;s assigned slots. The mobile station then analyzes the preliminary search results and attempts to acquire the more desirable wireless communications systems that meets preliminary search criteria. The mobile station includes a communications transceiver that facilitates wireless communications with a local base station and processing circuitry adapted to control a slotted operation mode of the mobile station. The processing circuitry is adapted to instruct the communications transceiver to listen for incoming messages from the acquired communications system during slot modes and listen for candidate communications systems during slot-off modes. The preliminary searches include performing tests on the signal received on a candidate communications system channel. The tests may include measuring the signal strength, calculating the ratio E c /I o  and demodulating a synchronization signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to wirelesscommunications and, in particular, to a method and apparatus foridentifying and acquiring a wireless communications system that is moredesirable than a current wireless communications system.

[0003] 2. Description of the Related Art

[0004] A mobile device may have access to more than one wirelesscommunications system in its current geographic region. The quality ofthe wireless services available to the mobile device will vary fromsystem to system depending on the equipment used by each system, thefeatures of the mobile device, the distance between the mobile deviceand local base stations, physical obstructions such as buildings andhills, and the volume of communications traffic on each system. Thewireless communications systems may also support differentmultiple-access wireless communications protocols such as code divisionmultiple access (CDMA), wideband code division multiple access (WCDMA),time division multiple access (TDMA), Advanced Mobile Phone System(AMPS) or Global System for Mobile Communications (GSM). In addition,the fees charged to a user of the mobile device often vary depending onthe time of day, the duration of the connection with the wirelesscommunications system and whether the mobile device is listed as asubscriber of the wireless communications system.

[0005] To assist with the selection of a wireless communications system,conventional mobile devices store data describing each known wirelesscommunications system in a systems table. Each entry in the systemstable includes a system identifier (SID), a network identifier (NID),and acquisition parameters including band, frequency and mode. Thewireless communications systems are often grouped by geographic region,and sorted from the most desirable to the least desirable system in eachregion. The most desirable system in a particular geographic region istypically a subscriber system, but may also be a roaming system thatoffers the mobile device the best combination of low cost and highquality of service. Roaming systems offer wireless services tonon-subscriber mobile devices, usually at a much higher fee thansubscription services, and may be desirable when the mobile deviceenters a geographic region that is outside the coverage area of themobile device's subscription service, when the subscription services areblocked or otherwise unavailable, or when the subscription services areonly available at an unacceptably low level of quality.

[0006] When activated, a conventional mobile device attempts to acquirethe most desirable wireless communications system that is available inthe current geographic region. The mobile device may determine itscurrent geographic region by acquiring a first wireless communicationssystem and locating its corresponding entry in the systems table. In oneapproach, the mobile device maintains a table of the wirelesscommunications systems that were recently used by the mobile device, andthe mobile device attempts to acquire one of these systems uponactivation. In another approach, the mobile device initially attempts toacquire the wireless communications systems that are known to be theeasiest to locate. For example, some wireless communications systems indifferent geographic regions use the same acquisition parameters,increasing the likelihood that the mobile device will be able to acquirea system when using these parameters, regardless of the currentgeographic region. Once a wireless communications system is acquired,the mobile device searches for the SID/NID pair of the acquired systemin the systems table to determine its corresponding geographic region.The mobile device then searches the systems table for the most desirablesystem in the identified geographic region and attempts to acquire thatsystem. If the mobile device is unable to acquire the most desirablesystem, the mobile device steps through the other entries in the currentgeographic region, from the most desirable to the least desirable, untila wireless communications system is acquired.

[0007] The use of a systems table to acquire the most desirable systemdoes not guarantee that the mobile device will be always be connected tothe most desirable system that is available in its current geographicregion. During system acquisition, for example, the most desirablesystem may have been temporarily blocked or weakened by physicalobstructions, forcing the mobile device to acquire a less desirablesystem. Even if the most desirable system is initially acquired, thesystems that are available to the mobile device may change as theposition of the mobile device and its surrounding environment changes.To provide the user of the mobile device with the best combination ofhigh quality and low cost, some mobile devices periodically identify andattempt to acquire a wireless communications system that is moredesirable than the current wireless communications system used by themobile device. In operation, the mobile device searches the systemstable for wireless communications systems in the current geographicregion that are more desirable than the current communications system.If more desirable systems are found, the mobile device switches awayfrom the current communications channel and attempts to acquire one ofthe more desirable systems.

[0008] Although it is beneficial to communicate using the most desirablewireless communications system that is available to the mobile device inthe current geographic region, the approach describe above has manydrawbacks. For example, it is common for the mobile device to leave thecurrent communications channel for up to 10 seconds every 60 secondswhile attempting to acquire a more desirable system. The duration oftime that the mobile device is away from the current communicationschannel will depend on many factors, including the number of searchesthat must be performed in order to acquire a more desirable system andthe communications protocol used by each of the wireless communicationssystems. While the mobile device is searching for wirelesscommunications systems on other channels, the mobile device may missimportant pages, telephone calls and other incoming communications fromthe current wireless communications system. In addition, this frequentand prolonged switching away from the current wireless communicationssystem increases the likelihood that the current system will be lostwhen the signal is weak. Further, the numerous acquisition attempts thatare performed in this approach drain the battery power of the mobiledevice, reducing its standby time.

[0009] In view of the above, there is a need in the art for an improvedmethod and apparatus for identifying and acquiring a wirelesscommunications system that is more desirable than a current wirelesscommunications system.

SUMMARY OF THE INVENTION

[0010] The present invention is an improved method and apparatus foridentifying and acquiring a wireless communications system that is moredesirable than the communications system currently used by a mobilestation. In one embodiment, a mobile station performs preliminarysearches for more desirable wireless communications systems in betweenpaging channel and quick paging channel assigned slots whilesuccessfully monitoring the mobile station's assigned slots. The mobilestation may then attempt to acquire the more desirable systems thatsatisfy certain preliminary search criteria.

[0011] In a preferred embodiment, a mobile station includes a systemstable that stores a list of wireless communications systems that areknown to the mobile station. Each record in the systems table includesdata that describes a unique SID/NID pair of a corresponding wirelesscommunications system. The mobile station also stores acquisitionparameters for each wireless communications system, such as band,frequency and mode, an identifier of the system's correspondinggeographic region and a relative desirability of the wirelesscommunications system.

[0012] In operation, the mobile station establishes a wirelesscommunications link with a local base station and periodicallydetermines whether the current communications system is the mostdesirable system in the current geographic region. The mobile station'scurrent geographic region may be identified by locating the SID and NIDreceived by the base station in the systems table. The mobile stationmay then search the systems table for candidate systems in the samegeographic region and that have a relative desirability that is greaterthan the relative desirability of the current system. If at least onecandidate system is found, the mobile device analyzes the candidatesystem during a slot-off period, while continuing to monitor the slotperiod assigned by the base station. Based on the analysis of thecandidate system, the mobile station determines whether the candidatesystem is likely or unlikely to be acquired by the mobile station. Themobile station then attempts to acquire the candidate systems that arelikely to be acquired.

[0013] The mobile station preferably includes processing circuitry, amemory, a communications transceiver and an antenna. The processingcircuitry includes a control processor that controls the mobilestation's slotted sleep mode, a signal processor that processescommunications signals, a searcher and a system determination unit. Thememory includes both volatile and nonvolatile random access memoriesthat store a systems table, an acquisition table, and at least onelookup table. The communications transceiver is preferably adapted toreceive and transmit both analog and digital signals such as AMPS andCDMA, respectively, through the antenna. The searcher is adapted toidentify valid signals, such as pilot signals and paging channels,received by the transceiver, and to perform a preliminary acquisitionanalysis of one or more candidate communications systems. Thepreliminary acquisition analysis may be performed in a dedicated mode orin a background mode during slot-off periods. The system determinationunit is adapted to determine whether the current communications systemis the most desirable system in the current geographic region and, whenmore desirable systems are available, initiate attempts by the mobilestation to acquire a more desirable system.

[0014] When activated, the mobile station attempts to acquire the mostdesirable wireless communications system that is available in the mobilestation's current geographic region. The local base station of theacquired wireless communications system uses a paging channel, and insome embodiments a quick paging channel, to notify the mobile station ofincoming messages, such as an incoming telephone call or a receivedvoicemail message. When the mobile station is idle, it enters a slottedsleep mode during which the mobile station cycles between a slot-offperiod and a slot period. During the slot-off period, the mobile stationshuts down the transceiver and other electronics to preserve batterypower. The mobile station wakes up at predetermined time intervals(i.e., slot periods), turns on the transceiver and listens for incomingmessage on the paging channel. If no messages are received that requirethe mobile station to stay awake, the mobile station goes back to sleepduring the next slot-off period.

[0015] The initially acquired wireless communications system may not bethe most desirable system available in the mobile station's currentgeographic region. In a preferred embodiment for identifying andacquiring a wireless communications system that is more desirable thanthe current communications system the mobile station first determineswhether the current communications system is the most desirable systemavailable in the current geographic region. If at least one moredesirable system is found then the mobile station determines thelikelihood that each of the more desirable systems will be acquired.This determination is preferably performed during one or more slot-offperiods. If at least one more desirable system is likely to be acquired,then the mobile station attempts to acquire one of the more desirablesystems.

[0016] To determine the likelihood that a candidate communicationssystem will be acquired, the mobile station performs one or more testson the candidate communications system. In a preferred embodiment, thesystem determination unit selects one or more candidate communicationssystems and the searcher tests each candidate communications systemduring one or more slot-off periods. In operation, the searcher waitsfor a slot-off period, selects a candidate system, switches to thechannel of the selected system and tests the received signal. In a firstembodiment, the test performed by the searcher includes measuring thereceived signal strength (Rx) during the slot-off period. If thereceived signal strength measurement is relatively low, there would be alow probability that the candidate system would be available foracquisition. If the measurement is relatively high, there would be agreater likelihood that the candidate system could be acquired. If thereis one or more candidate systems remaining to be tested, the timeremaining in the slot-off period is checked to determine whether anothertest can be performed within the slot-off period. If an additional testcan be performed, then the next candidate system is selected and tested.If the time remaining in the slot-off period is insufficient to testanother candidate system, the mobile station returns to the currentcommunications channel to monitor the next slot period, and waits for asubsequent slot-off period before selecting and testing the nextcandidate system. When there are no more candidate systems to test, thesearcher switches back to the current communications channel before theexpiration of the current slot-off period. In alternate embodiments, thetesting of candidate systems may terminate after a predetermined numberof candidate systems meet a minimum testing threshold, when interruptedby the control processor or system determination unit, when the mobilestation changes states or other conditions are satisfied.

[0017] In a preferred embodiment, the candidate systems that have acorresponding measured signal strength lower than a threshold value(e.g., −90 db) are eliminated from the candidate systems list, and theremaining candidate systems are then sorted in order of desirability.Other criteria may also be used to eliminate and sort candidate systems.The mobile station then switches to the channel of the first candidatesystem and attempts to acquire the corresponding system. Any systemacquisition method may be used including conventional system acquisitionmethods that are well-known in the art. If the more desirable system isfound, the mobile station may register with the new system and use itfor future communications. If the more desirable system is not found,the mobile station will attempt to acquire the next system from thecandidate systems list. This process continues until a more desirablesystem is acquired, or until there are no additional candidate systems.If the mobile station fails to acquire a new system, then the mobilestation will attempt to return to the previous communications system.

[0018] Other tests may also be performed on the candidate systemsprovided that the tests are capable of being performed within one ormore slot-off periods and the test results assist in determining whetherthe candidate systems are likely to be acquired. In a first alternateembodiment, the analysis of the candidate systems includes demodulatingthe received signal to determine whether it is likely to be CDMA signal.In this embodiment, the mobile station measures the signal to noiseratio E_(c)/I_(o), where E_(c) is the strength of the received signaland I_(o) is the total thermal noise received on the channel. If theratio E_(c)/I_(o) is relatively large, then there is a high probabilitythat the CDMA system could be acquired. In a second alternateembodiment, the analysis of the candidate systems includes attempting todecode the synchronization messages of the candidate systems. If asynchronization message is decoded, the mobile station will obtain thetransmitting base station's SID/NID pair. If the obtained SID/NID pairmatches the SID/NID pair of the candidate system, then there is a stronglikelihood that the candidate system may be acquired. In a thirdalternate embodiment, the mobile station supports multiple modes ofmeasurement and acquisition, and may be adapted to use any of theapproaches described above, alone or in combination. The tests to beperformed on the candidate systems may be determined by userpreferences, configuration of the mobile station, current operatingstate of the mobile station or other criteria. In addition, the analysisof candidate systems may be selectively performed in the background(during slot-off periods) or in a dedicated mode depending on the stateof the mobile station.

[0019] A more complete understanding of the Method and Apparatus forIdentifying and Selecting Preferred Wireless Communications Systems willbe afforded to those skilled in the art, as well as a realization ofadditional advantages and objects thereof, by a consideration of thefollowing detailed description of preferred embodiments. Reference willbe made to the appended sheets of drawings, which will first bedescribed briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0021]FIG. 1 illustrates a preferred operating environment of thepresent invention;

[0022]FIG. 2 is a preferred embodiment of a mobile station;

[0023]FIGS. 3a and 3 b illustrate preferred embodiments of a systemtable and an acquisition table;

[0024]FIG. 4 illustrates a preferred embodiment of a lookup tablememory;

[0025]FIG. 5 is a flow diagram illustrating a preferred algorithm foracquiring a more desirable wireless communications system; and

[0026]FIG. 6 is a flow diagram illustrating a preferred algorithm fordetermining whether a more desirable system is likely to be acquired.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0027] A preferred embodiment of the present invention is illustrated inFIGS. 1-6. Referring to FIG. 1, a geographic region 2 includes aplurality of base stations 4 that provide wireless services to localmobile stations, such as mobile station 6. Each base station 4 isconnected to a network 8, which is part of a larger wirelesscommunications system that supports at least one multiple-accesswireless communications protocol such as code division multiple access(CDMA), wideband CDMA (WCDMA), time division multiple access (TDMA),frequency division multiple access (FDMA), Advanced Mobile Phone Service(AMPS), Global System for Mobile communications (GSM), PersonalCommunication Systems (PCS), General Packet Radio Services (GPRS) orHigh Data Rate (HDR) technology (e.g., 1×EV technology). The mobilestation 6 may be any wireless device, whether stationary or mobile, thatis adapted for wireless communications with at least one of the basestations 4, such as a cellular telephone, pager, personal digitalassistant (PDA), vehicle navigation system or portable computer.

[0028] In a preferred embodiment, the mobile station 6 and at least onebase station 4 are further adapted for slotted communications on pagingand quick paging channels as known in the art. In slotted operation, themobile station 6 cycles between a slot-off period and a slot period.During the slot-off period, the mobile station 6 shuts down its receiverand other electronics to preserve battery power during idle periods.During the slot period, the mobile station 6 turns on its receiver andlistens for incoming messages from the base station 4.

[0029] The mobile station 6 includes a systems table 10 that stores alist of known wireless communications systems. In a preferredembodiment, each wireless communications system has a unique systemidentifier (SID) and, within each wireless communications system, eachnetwork 8 has a unique network identifier (NID). Each record in thesystems table 10 stores data that describes a unique SID/NID pair,including corresponding acquisition parameters such as band, frequencyand mode, an identifier of the system's corresponding geographic region,and a relative desirability of the wireless communications system. It iscontemplated that, in alternative embodiments, other identification andacquisition parameters may be used in accordance with the presentinvention.

[0030] In a preferred operation, the mobile station 6 establishes awireless communications link with one of the base stations 4 andperiodically determines whether the corresponding wirelesscommunications system is the most desirable system in the currentgeographic region 2. The mobile station's current geographic region 2may be identified by locating the SID and NID of the currentcommunications system in the systems table 10. The mobile station 6 thensearches the systems table 10 for candidate systems from the samegeographic region that have a relative desirability that is greater thanthe relative desirability of the current system. If at least onecandidate system is found, the mobile device 6 analyzes the candidatesystem during a slot-off period, while continuing to monitor the slotperiod assigned by the current communications system. Based on theanalysis of the candidate system, the mobile device 6 may determinewhether the candidate system is likely or unlikely to be acquired by themobile station 6. If acquisition is likely, the mobile station 6attempts to acquire the candidate system.

[0031] Referring to FIG. 2, a preferred embodiment of a mobile station20 will now be described. The mobile station 20 includes processingcircuitry 22, a memory 24, a communications transceiver 26 and anantenna 28. The processing circuitry 22 preferably includes a controlprocessor 30, a signal processor 32, a searcher 34 and a systemdetermination unit 36. The control processor 30 controls the operationof the mobile station 20, including the operation of a slotted sleepmode. The memory 24 includes both volatile and nonvolatile random accessmemories that store a systems table 38, an acquisition table 40 and atleast one lookup table 42. In a preferred embodiment, the communicationstransceiver 26 is adapted to receive and transmit both analog anddigital signals such as AMPS and CDMA, respectively, through the antenna28.

[0032] The searcher 34 is adapted to identify valid signals, such aspilot signals and paging channels, received by the transceiver 26. Thedesign and implementation of searcher hardware for CDMA acquisition isdescribed in U.S. Pat. No. 5,109,390, entitled “DIVERSITY RECEIVER IN ACDMA CELLULAR TELEPHONE SYSTEM,” assigned to the assignee of the presentinvention and incorporated herein by reference. The searcher 34 isfurther adapted to perform a preliminary acquisition analysis of one ormore candidate communications systems. In a preferred embodiment, thepreliminary acquisition analysis may be performed in a dedicated mode,or in a background mode during slot-off periods.

[0033] The system determination unit 36 is adapted to select one or morewireless communications systems from the systems table 38, and retrievecorresponding acquisition parameters from the acquisition table 40. Thesystem determination unit 36 transmits the acquisition parameters to thesearcher 34, which attempts to acquire one the selected systems. Thesystem determination unit 36 preferably communicates with the controlprocessor 30 and the searcher 34 through function calls. In a preferredembodiment, the control processor 30 communicates changes in mode, band,frequency, SID and NID to the system determination unit 36. Otherfunction calls may relate to roaming preferences, emergency 911services, and over-the-air service provisioning (OTASP). The systemdetermination unit 36 is also adapted to determine whether the currentcommunications system is the most desirable system in the currentgeographic region and, when more desirable systems are available,initiate attempts by the mobile station 20 to acquire a more desirablesystem.

[0034] It should be appreciated that the mobile station 20 illustratedin FIG. 2 is merely illustrative and that alternative configurations andadditional features are contemplated within the scope and spirit of thepresent invention. For example, the components of the mobile station 20may be implemented in numerous hardware configurations usingconventional circuit elements such as one or more processors, memoriesand application specific integrated circuits (ASICs). The mobile station20 may also be adapted for voice communications, high-speed datacommunications, video communications, internet applications such asemail and World Wide Web access, position location (e.g., gpsOne™developed by assignee), personal navigation, voice recognition,integrated removable storage devices and short-range wirelessconnectivity with local peripherals and devices. In a preferredembodiment, the processing circuitry 22 includes a Mobile Station Modemchipset, developed by assignee, which integrates digital and analogfunctions along with GPS-based position location.

[0035] Referring to FIGS. 3a and 3 b, preferred embodiments of thesystem table 38 and the acquisition table 40 will now be described. Thesystem table 38 includes one record for each known wirelesscommunications system, including both preferred and negative systems. Awireless communications system is “known” if the mobile station 20 hasattempted to acquire the system or if system information was transmittedto the mobile device 20 from an external source. In the system table 38,the wireless communications systems are preferably grouped by geographicregion and stored in sorted order from the most desirable system in theregion to the least desirable system in the region. The desirability ofeach system may be determined using criteria such as the cost of usingthe system, quality of the communications service, protocol used by thesystem, support for unique features and whether the mobile station 20 isa subscriber of the wireless communications system. As illustrated, eachrecord in the system table 38 includes a system identifier (SID),network identifier (NID), an indication of whether the system ispreferred or negative (i.e., for emergency use only), an identifier ofthe geographic region (GID) and a pointer to a record in the acquisitiontable 40. Other information may also be stored in the system table 38,such as a desirability rating (e.g., a numerical score) for each system.Each record in the acquisition table 40 includes a band, frequency, modeand any additional parameters that are necessary for acquisition of thesystems listed in the system table 38. The system table 38 and theacquisition table 40 are preferably stored in a nonvolatile memory andupdated periodically through downloads across a wireless connection fromthe subscription service or by another data transfer method.

[0036] As illustrated in FIG. 4, the memory 24 also includes one or morelookup tables. In a preferred embodiment the lookup tables include amost recently used (MRU) systems table 44 that stores a list of thesystems most recently used by the mobile station 20, an availablesystems table 46 that includes systems that are likely to be availablebased on recent acquisition attempts, a candidates list 48 that includesa list of local systems that are more desirable than the currentcommunications system, and a preferences table 50 that maintains localpreference settings for the mobile station 20. Other information mayalso be stored in a lookup table including historical information thattracks the last time a system was acquired. The lookup tables arepreferably stored in volatile portion of the memory 24, but in alternateembodiments, one or more of the lookup tables may be stored in anonvolatile portion of the memory 24.

[0037] A preferred operation of the mobile station 20 will now bedescribed. When activated, the mobile station 20 attempts to acquire themost preferred wireless communications system that is available in themobile station's current geographic region. In a preferred embodiment,the system determination unit 36 selects a system from the MRU systemstable 44 and instructs the searcher 24 to attempt acquisition of theselected system. The mobile station 20 is preferably a dual mode devicethat is adapted to acquire both CDMA and AMPS systems. To acquire a CDMAsystem, the mobile station 20 switches to the communications channel ofthe selected CDMA system and listens for a pilot signal. The searcher 34attempts to verify the received pilot signal by testing variouspseudorandom noise (PN) offsets in the received signal until a match isfound. When the pilot signal is acquired, the mobile station 20 receivesinformation for the forward CDMA channel and a phase reference forsignal demodulation. Next, the mobile station 20 attempts to acquire thesynchronization channel associated with the identified pilot channel.The synchronization channel transmits basic system information such asthe unique SID/NID of the transmitting wireless communications systemand network, and synchronization information. The mobile station 20adjusts its timing in accordance with the received information and thenswitches to the base station's paging channel. The paging channel isused by a local base station of the current communications system tocommunicate with the mobile station 20 when the mobile station 20 is notassigned to a traffic channel. Through the paging channel, the basestation notifies the mobile station 20 of incoming messages, such as anincoming telephone call or a notification of a received voicemailmessage.

[0038] If the searcher 34 is unable to acquire the selected system, thesearcher 34 will preferably attempt to acquire each of the remainingsystems stored in the MRU systems table 44, and subsequently, thesystems stored in the systems table 38, until a wireless communicationssystem is acquired. After a system is acquired, the system determinationunit 36 searches for the acquired system's SID/NID pair in the systemstable 38 to determine the acquired system's geographic region. Thesystem determination unit 36 then searches the systems table 38 forwireless communications systems in the same geographic region as theacquired system that are more desirable than the acquired communicationssystem. The system determination unit 36 then instructs the searcher 34to acquire one of the more desirable systems, in order of desirability.If the searcher 34 is unable to acquire a more desirable system, themobile device 20 may use the initially acquired wireless communicationssystem for future wireless communications. It will be appreciated thatany method for acquiring a wireless communications system may be used inaccordance with the present invention. A method and apparatus forperforming preferred system selection in a mobile station that iscapable of operation in a plurality of geographic regions is disclosedin U.S. Pat. No. 6,085,085, entitled “METHOD AND APPARATUS FORPERFORMING PREFERRED SYSTEM SELECTION,” assigned to assignee, and isincorporated herein by reference.

[0039] When the mobile station 20 is idle, it preferably enters aslotted sleep mode during which the mobile station 20 cycles between aslot-off period and a slot period. During the slot-off period, themobile station 20 shuts down the transceiver 26 and other electronics topreserve battery power. The mobile station 20 wakes up at predeterminedtime intervals (i.e., slot periods), turns on the transceiver 26 andlistens for incoming message on the paging channel. If no messages arereceived that require the mobile station 20 to stay awake, the mobilestation 20 goes back to sleep during the next slot-off period.

[0040] The current wireless communications system acquired and used bythe mobile station 20 may not be the most desirable system available inthe mobile station's current geographic region. For example, even if themobile station 20 acquired the most desirable system that was availableat the time of acquisition, the wireless communications systemsavailable to the mobile station 20 are likely to change as the positionof the mobile device 20 and its surrounding environment changes. A moredesirable system may have been temporarily blocked or weakened due tophysical obstructions or may have been out of range at the time ofacquisition.

[0041] Referring to FIGS. 2 and 5, a preferred embodiment for acquiringa wireless communications system that is more desirable than the currentcommunications system will now be described. In Step 100, the mobilestation 20 determines whether the current communications system is themost desirable system available in the current geographic region. Thisdetermination is preferably made by the system determination unit 36,which locates the current SID/NID pair in the systems table 38 and thensearches the systems table 38 for systems in the same geographic regionthat are more desirable than the current system. The results of thissearch are stored in a candidates list. If at least one more desirablesystem is found then, in Step 102, the system determination unit 36instructs the searcher 34 to determine the likelihood that each of thecandidate systems will be acquired. This determination is preferablyperformed during one or more slot-off periods. In Step 104, if at leastone of the candidate communications systems is likely to be acquired,the system determination unit 36 instructs the searcher 34 to attemptacquisition of one of the candidate systems in Step 106. In a preferredembodiment, the algorithm illustrated in FIG. 5 is initiated by thesystem determination unit 36 in response to a change in the mobilestation's state information, such as after the current communicationssystem changes. For example, the algorithm may be implemented after ahandoff between based stations as the mobile station 20 travels betweencells. In an alternate embodiment, the algorithm of FIG. 5 is executedperiodically to ensure that the mobile station is using the mostdesirable system available.

[0042] Referring to FIGS. 2 and 6, a preferred algorithm for determiningthe likelihood of acquiring a candidate system (FIG. 5, Step 102) willnow be described. In Step 120, the searcher 34 waits for the mobilestation 20 to enter a slot-off period. In Step 122, the searcher 34selects a more desirable system from the candidates list. In a preferredembodiment, the candidates list is a lookup table stored in a memorythat includes each of the more desirable systems selected by the systemdetermination unit 36. In an alternate embodiment, the candidates listmay be transmitted to the searcher 34. In Step 124, the transceiver 26switches to the channel of the selected system, and then the searcher 34tests the received signal in Step 126. In a preferred embodiment, thetest performed by the searcher 34 includes measuring the received signalstrength (Rx) during the slot-off period. If the received signalstrength measurement is relatively low, there would be a low probabilitythat the candidate system would be available for acquisition. If themeasurement is relatively high, there would be a greater probabilitythat the candidate system could be acquired. In alternate embodiments,other tests may be performed on the received signal provided that thetests are capable of being performed within a slot-off period and thetest results assist in determining whether the candidate system islikely to be acquired.

[0043] If there is at least one more candidate system to test (Step128), the time remaining in the slot-off period is checked in Step 130to determine whether another test can be performed within the slot-offperiod. If an additional test can be performed within the slot-offperiod, then control is passed back to Step 122 and the next candidatesystem is selected and tested. If the time remaining in the slot-offperiod is insufficient to test another candidate system, the mobilestation 20 returns to the current communications channel in Step 132 tomonitor the next slot period, and waits for a subsequent slot-off periodin Step 120. When there are no more candidate systems to test, thesearcher 34 switches back to the current communications channel in Step134 before the expiration of the current slot-off period. In alternateembodiments, the algorithm of FIG. 6 may be terminated after apredetermined number of candidate systems meet a minimum measurementthreshold, when interrupted by the control processor or systemdetermination unit, when the mobile station changes states, or whenother criteria is met.

[0044] Referring back to FIG. 5, in Step 104 candidate systems that havea corresponding measured signal strength lower than a threshold value(e.g., 90 db) are eliminated from the candidates list. In a preferredembodiment, the remaining candidate systems are then sorted in order ofdesirability. Other criteria may also be used to eliminate and sortcandidate systems. For example, in a first alternate embodiment, thecandidate systems may be sorted in order of measured strength. In asecond alternate embodiment, a weighting factor is used that takes intoaccount the measured signal strength and whether the system was recentlyused. In a third alternate embodiment, a predetermined number ofcandidate systems that have the highest measured strength are retained,and the rest of the candidate systems eliminated from the candidateslist.

[0045] The mobile station next attempts to acquire one of the remainingcandidate systems. The mobile station switches to the channel of thefirst candidate system and attempts to acquire the corresponding system.Any system acquisition method may be used including conventional systemacquisition methods that are well-known in the art. If the moredesirable system is found, the mobile station may register with the newsystem and use it for future communications. If the more desirablesystem is not found, the mobile station will attempt to acquire the nextsystem from the candidates list. This process continues until a moredesirable system is acquired, or until there are no additional candidatesystems. If the mobile station fails to acquire a new system, then themobile station will attempt to return to the previous communicationssystem.

[0046] In an alternate embodiment, the test performed on the candidatesystem (FIG. 6, Step 126) includes demodulating the received signal todetermine whether it is likely to be CDMA signal. The systemdetermination unit 36 instructs the searcher 34 to measure the signal tonoise ratio E_(c)/I_(o), where E_(c) is the strength of the receivedsignal and I_(o) is the total thermal noise received on the channel. Ina CDMA system, this measurement provides an indication of the portion ofthe received signal that is usable. If the ratio E_(c)/I_(o) isrelatively large, then there is a high probability that the system couldbe acquired. The ratio E_(c)/I_(o) provides a more reliable indicationof whether the signal is likely to be acquired than the measured rawstrength of the signal. This measurement, however, takes longer tocalculate and it may require more slot periods to measure the potentialcandidate systems.

[0047] In a second alternate embodiment, the test performed in thecandidate system (FIG. 6, Step 126) includes attempting to decode thesynchronization message of the candidate system. If a synchronizationmessage is decoded, the mobile station 20 will obtain the base station'sSID/NID pair. If the obtained SID/NID pair matches the SID/NID pair ofthe candidate system, then there is a strong likelihood that thecandidate system will be acquired.

[0048] In a third alternate embodiment, the system determination unit 36and searcher 34 support multiple test modes and methods of acquisition,and may be adapted to use any of the approaches described above, aloneor in combination. The test mode may be determined by user preference,configuration of the mobile station, current operating state of themobile station or other criteria. In addition, the testing of candidatesystems may be performed in the background (during slot-off periods) orin a dedicated mode depending on the state of the mobile station. Forexample, during power up and initial system selection, the signalstrength of the available systems may be measured in a dedicated modebefore any acquisition attempts are made, to eliminate systems that areunlikely to acquired.

[0049] In a preferred embodiment, the searcher 34 operates in accordancewith instructions received from the system determination unit 36. Thesystem determination unit 36 is preferably adapted to transmit acandidate systems test instruction and a system acquisition instruction.The candidate systems test instruction includes parameters to identifythe candidate list, an identifier of the test to be performed on eachcandidate system, a minimum threshold value to be met by each system,the number of successful tests before returning the results to thesystem determination unit 36, and whether the test should be performedin dedicated or background mode. The searcher 34 executes the receivedinstruction and returns the candidate systems that meet the testingrequirements. In a preferred embodiment, the searcher 34 modifies thecandidates list stored in memory. The system determination unit 36 isfurther adapted to analyze the test results and transmit a systemacquisition instruction to the searcher 34, including parametersidentifying the candidates lists.

[0050] Having thus described a preferred embodiment of the Method andApparatus for Identifying and Acquiring a Preferred WirelessCommunications System, it should be apparent to those skilled in the artthat certain advantages of the within described system have beenachieved. It should also be appreciated that various modifications,adaptations, and alternative embodiments thereof may be made within thescope and spirit of the present invention.

[0051] The scope of the present invention is defined by the followingclaims.

1. In a mobile station adapted for slotted operation on a currentwireless communications system, a method for acquiring a wirelesscommunications system comprising the steps of: during a slot-off period,analyzing a signal received on a channel associated with a candidatecommunications system; determining, based on the analysis of thereceived signal, whether the candidate communications system is likelyto be available for acquisition by the mobile station; and attempting toacquire the candidate communications system if it is determined that thecandidate communications system is likely to be available foracquisition.
 2. The method of claim 1 further comprising the step ofselecting a set of candidate communications systems, wherein the step ofanalyzing is repeated for each candidate communications system in theset.
 3. The method of claim 2 wherein, for each candidate communicationssystem in the set, the step of analyzing comprises the steps of:switching to a channel associated with the candidate communicationssystem; and testing the received signal.
 4. The method of claim 3wherein, for each candidate communications system in the set, the stepsof switching and testing are completed during a single slot-off period.5. The method of claim 3 wherein, if the time remaining in the currentslot-off period is insufficient to complete the steps of switching andtesting for the current candidate communications system, the mobilestation returns to the current communications system until a subsequentslot-off period, and wherein the step of analyzing is resumed during asubsequent slot-off period.
 6. The method of claim 5 wherein the step ofanalyzing terminates when a predetermined condition is satisfied.
 7. Themethod of claim 6 wherein the step of analyzing terminates when apredetermined number of received signals satisfy predetermined testingcriteria.
 8. The method of claim 2 wherein each candidate communicationssystem is more desirable than the current communications system.
 9. Themethod of claim 2 wherein the mobile station includes a table of knowncommunications systems, each known communications system having arelative desirability, and wherein the set of candidate communicationssystems is selected from the table of known communications systems, andwherein each candidate communications system in the set has a relativedesirability that is greater than the current communications system. 10.The method of claim 9 wherein a geographic region for eachcommunications system is stored in the table of known communicationssystems and wherein the set of candidate communications systems includesonly known communications systems in the same geographic region as thecurrent communications system.
 11. The method of claim 2 wherein the setof candidate communications systems includes at least one digitalcommunications system and at least one analog communications system. 12.The method of claim 1 wherein the step of analyzing comprises the stepof measuring the strength of the received signal.
 13. The method ofclaim 12 wherein the candidate communications system is expected to beavailable if the measured strength of the received signal exceeds apredetermined threshold value.
 14. The method of claim 13 furthercomprising the step of selecting a set of candidate communicationssystems, wherein the steps of analyzing and determining are repeated foreach candidate communications system in the set.
 15. The method of claim14 wherein the step of attempting to acquire is performed for eachcandidate communications system that has a corresponding measured signalstrength that exceeds the predetermined threshold value, until acandidate communications system is acquired or the candidatecommunications systems are exhausted.
 16. The method of claim 15 whereinthe step of determining comprises the step of sorting the candidatecommunications systems in order of measured strength, the sorted orderdefining the order of attempted acquisition.
 17. The method of claim 15wherein the step of determining comprises the step of sorting thecandidate communications systems in order of desirability, the sortedorder defining the order of attempted acquisition.
 18. The method ofclaim 1 wherein the step of analyzing comprises the step of calculatinga ratio E_(c)/I_(o) of the received signal.
 19. The method of claim 18wherein the candidate communications system is expected to be availableif the calculated ratio E_(c)/I_(o) exceeds a predetermined thresholdvalue.
 20. The method of claim 1 wherein the step of analyzing comprisesthe step of attempting to decode the received signal.
 21. The method ofclaim 1 wherein the step of analyzing comprises the step of locating aSID and a NID in the received signal, and wherein the step ofdetermining comprises the step of verifying that the located SID and NIDmatch a SID and NID of the candidate communications system.
 22. A mobilestation comprising: a communications transceiver that facilitateswireless communications with a base station of a current wirelesscommunications system; a processing circuitry coupled to thecommunications transceiver, the processing circuitry controlling aslotted operation mode of the mobile station, the slotted operation modeincluding: a slot period during which the processing circuitry instructsthe communications transceiver to listen for incoming messages from thecurrent communications system, and a slot-off period during which theprocessing circuitry instructs the communications transceiver to listenfor a candidate communications system.
 23. The mobile station of claim22 further comprising: a memory coupled to the processing circuitry, thememory storing a system table that includes a list of wirelesscommunications systems, each wireless communications system having arelative desirability and an associated geographic region, wherein therelative desirability of the candidate communications system is greaterthan the relative desirability of the current communications system. 24.The mobile station of claim 23 wherein the processing circuitry includessearcher logic to analyze a signal received during the slot-off periodand determine, based on the analysis, whether the candidatecommunications system is likely to be available for acquisition by themobile station.
 25. The mobile station of claim 24 wherein theprocessing circuitry initiates an attempt to acquire the candidatecommunications system if it is determined that the candidatecommunications system is likely to be available for acquisition.
 26. Ina wireless device, an integrated circuit comprising: a control processorincluding logic for controlling a slotted operation mode of the wirelessdevice, the slotted operation mode including: a slot period during whichthe control processor instructs the wireless device to listen forincoming messages from a current wireless communications system, and aslot-off period during which the control processor instructs thewireless device to enter a sleep mode; a system determination unitcoupled to the control processor, the system determination adapted toidentify candidate communications systems in a current geographic regionof the wireless device that are more desirable than the current wirelesscommunications system; and a searcher coupled to the control processorand system determination unit, the searcher adapted to analyze a signalquality of at least one of the identified more desirable communicationssystems, wherein the measurement of each more desirable communicationssystem is performed during a single slot-off period.
 27. The integratedcircuit of claim 26, wherein the searcher measures the strength of areceived signal for each identified more desirable wirelesscommunications system.
 28. The integrated circuit of claim 26, whereinthe searcher calculates the ratio E_(c)/I_(o) of a received signal foreach identified more desirable wireless communications system.
 29. Theintegrated circuit of claim 26 further comprising a memory coupled tothe system determination unit, the memory storing a list of knowncommunications systems, each known communications system having anassociated geographic region and relative desirability; wherein thesystems analyzed by the searcher are selected from the system table. 30.The integrated circuit of claim 29 wherein the searcher is adapted toanalyze the signal quality of each identified candidate communicationssystem and notify the system determination unit of the results.
 31. Theintegrated circuit of claim 26 wherein the system determination unit isadapted to transmit an instruction to the searcher, the instructionincluding a test identifier; and wherein, in response to a receivedinstruction, the searcher analyzes the signal quality of at least one ofthe identified more desirable communications systems using a test methodidentified by the test identifier.
 32. The integrated circuit of claim31 wherein the transmitted instruction further includes a thresholdvalue, and wherein the searcher transmits a notification message to thesystem determination unit when the analyzed signal quality exceeds thethreshold value.